nacho4d nacho4d - 1 year ago 86
Swift Question

Calling a generic function in swift

I have the following struct, variable and a function:

struct MyModel {
var keyString: String
var keyNum: Int

let data = "{\"keyString\": \"valueString\", \"keyNum\": 1 }"

func myFunction<T: AnyObject>(str: String) throws -> T? {
return nil

How can I call the function with MyModel? Below code makes the compiler complain: "Generic parameter 'T' could not be inferred"

let myModel = try? myFunction(str: data) as? MyModel

Link to Swift REPL:


Thanks to @Sweeper this is what I came out with
(Swift Sandbox Code):

protocol Initializable {

struct MyModel : Initializable {
var keyString: String = "ten"
var keyNum: Int = 10
public init() {

func stringToModel<T: Initializable>(str: String) throws -> T? {
return T() // TODO

func main() {
let maybeModel = (try? stringToModel(str: "whatever") as MyModel?) ?? nil
guard let model = maybeModel else {
print("keyNum \(model.keyNum) should be 10")


Answer Source

Since your generic type parameter is declared with a constraint, like this:

func myFunction<T: AnyObject>

So whatever type you use, it must be a reference type or implement NSObjectProtocol or something like that.

A swift struct is a value type, so MyModel can't possibly be used as a generic type parameter.


  • Change the generic type constraint to <T: Any> to make it accept value types.

  • Remove the generic type constraint.

  • Change the struct to a class and add an initializer.
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